U.S. patent number 5,606,555 [Application Number 08/445,755] was granted by the patent office on 1997-02-25 for apparatus and method for operating and constructing an optical tdm/tdma system having enhanced range.
This patent grant is currently assigned to Siemens Aktiengesellschaft. Invention is credited to Josef Singer.
United States Patent |
5,606,555 |
Singer |
February 25, 1997 |
Apparatus and method for operating and constructing an optical
TDM/TDMA system having enhanced range
Abstract
In conventional TDM/TDMA systems the signal blocks sent in TDMA
mode from a detached unit to a central station contain a control
data part whose length is derived from the signal running time in
the opposite direction in TDMA mode from the central station to the
detached unit at the largest distance therefrom. Any range
enhancement is such a known system then leads to a longer control
data part, and thus to a diminished transmission capacity for
useful signals. To achieve a range enhancement without diminishing
the transmission capacity, a TDM/TDMA system and a method for
constructing and operating such a system are disclosed wherein, the
detached units are divided into range zones and, except for the
units at the largest distance from the central station, the
detached units of respective range zones contain an additional,
further delay, so that the transmission cycle of the detached units
begins earlier than the reception cycle.
Inventors: |
Singer; Josef (Buchloe,
DE) |
Assignee: |
Siemens Aktiengesellschaft
(Munich, DE)
|
Family
ID: |
6518631 |
Appl.
No.: |
08/445,755 |
Filed: |
May 22, 1995 |
Foreign Application Priority Data
|
|
|
|
|
May 20, 1994 [DE] |
|
|
44 17 771.2 |
|
Current U.S.
Class: |
370/465; 370/442;
398/99; 398/98; 379/242 |
Current CPC
Class: |
H04J
3/0682 (20130101) |
Current International
Class: |
H04B
10/207 (20060101); H04J 3/06 (20060101); H04J
001/16 () |
Field of
Search: |
;370/56,58.1,58.2,77,85.2,95.3,79,108,110.1,13,24,29,95.1,100.1,106.1,105.6
;379/156,219,242 ;359/137,115,118,125,167,135,136,140 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
"Optische Ubertragungstechnik f ur fl achendeckende
Teilnehmeranschl usse (Teil 1)" Sporleader et al. Fernmelde
Ingenieur, vol. 4, Apr. 1992, pp. 9-12. .
"Optische Ubertragungstechnik f ur fl achendeckende
Teilnehmeranschl usse (Teil 2)" Sporleader et al. Fernmelde
Ingenieur, vol. 9, Sep. 1992, pp. 24-29. .
"Optische Ubertragungstechnik f ur fl achendeckende
Teilnehmeranschl usse (Teil 3)" Sporleader et al. Fernmelde
Ingenieur, vol. 10, Oct. 1992, pp. 4-6 and 11-13..
|
Primary Examiner: Safourek; Benedict V.
Assistant Examiner: Ngo; Ricky Q.
Attorney, Agent or Firm: Hill, Steadman & Simpson
Claims
We claim as our invention:
1. An optical TDM/TDMA system comprising:
central station means for emitting optical TDM signal blocks in a
predetermined first transmission cycle of TDM signal blocks in a
frame format and for receiving optical TDMA signal blocks;
a plurality of subscriber-proximate, detached means, each disposed
at a transmission distance from said central station means, for
receiving said TDM signal blocks from said central station means in
a reception cycle and for emitting said TDMA signal blocks in reply
to said TDM signal blocks, said plurality of detached means
collectively emitting said TDMA signal blocks in a second
transmission cycle of said TDMA signal blocks in said frame format
with each detached means emitting a signal block into said second
transmission cycle at a position in said second transmission cycle
dependent on the transmission distance of that detached means from
said central station means, said plurality of detached means
including at least two detached means respectively disposed at
different transmission distances from said central station
means;
an optical branch network connecting said central station means to
each detached means via which said TDM and TDMA signal blocks are
transmitted;
said central station means including means for defining an
anticipation window for each detached means during which a TDMA
signal from that detached means is expected to be present in said
second transmission cycle;
said plurality of detached means being divided into at least two
range zones of respectively different transmission distances;
all detached means in each range zone including delay means for
delaying emission of a TDMA signal block from that detached means,
following reception of a TDM signal block, by a delay dependent on
the range zone, with all detached means in a range zone at a
longest transmission distance having a first delay and all detached
means respectively in other range zones having further respective
delays longer than said first delay; and
means for introducing a frame offset between said first and second
transmission cycles equal to a duration of said anticipation window
multiplied by the plurality of range zones decremented by one, and
for initiating the second transmission cycle in reply to the first
transmission cycle only after completion of said reception
cycle.
2. An optical TDM/TDMA system as claimed in claim 1 wherein said
plurality of range zones comprises two range zones, and wherein
said first delay of all detached means in one of said two range
zones at a larger transmission distance being selected, given said
frame offset, so that a signal block emitted by a detached unit at
a lower limit of said range zone at said larger transmission
distance arrives at said central station just coinciding with a
beginning of said anticipation window.
3. An optical TDM/TDMA system as claimed in claim 2 wherein a first
of said range zones comprises all detached units respectively
having a transmission distance in a range of 0 through
approximately 11 km and wherein said second range zone comprises
all detached units having a transmission distance of approximately
9 through approximately 20 km.
4. A method for operating and constructing a TDM/TDMA system
comprising the steps of:
emitting optical TDM signal blocks in a predetermined first
transmission cycle of TDM signal blocks in a frame format from a
central station, and receiving optical TDMA signal blocks at said
central station;
receiving said TDM signal blocks collectively in a reception cycle
in a plurality of subscriber-proximate, detached units each
disposed at a distance from said central station;
emitting a second transmission cycle of said TDMA signal blocks in
said frame format collectively from said plurality of detached
units in reply to said first transmission cycle with each detached
unit emitting a signal block into said second transmission cycle at
a position in said second transmission cycle dependent on the
transmission distance of that detached unit from said central
station;
disposing at least two of said detached units at respectively
different transmission distances from said central station;
defining an anticipation window at said central station for each
detached unit during which a TDMA signal from that detached unit is
expected to be present in said second transmission cycle;
dividing said plurality of detached units into at least two range
zones of respectively different transmission distances;
delaying emission of said TDMA signal block from each detached unit
respectively in a range zone, following receipt of a TDM signal
block, by a delay dependent on the range zone, with all detached
units in a range zone at a largest transmission distance having a
first delay and all detached units respectively in other range
zones having respective further delays larger than said first
delay;
introducing a frame offset between said first and second
transmission cycles equal to a duration of said anticipation window
multiplied by the plurality of range zones decremented by one;
and
initiating said second transmission cycle in reply to said first
transmission cycle only after completion of said reception
cycle.
5. A method as claimed in claim 4 wherein the step of dividing said
detached units into at least two range zones comprises dividing
said plurality of detached units into two range zones, and
providing all detached units in one of said two range zones at a
larger transmission distance so that a signal block emitted by a
detached unit at a lower limit of said range zone just coincides
with a beginning of said anticipation window with said frame offset
taken into account.
6. A method as claimed in claim 5 wherein the step of dividing said
detached units into at least two range zones comprises dividing all
detached units into having a transmission distance in a range of 0
through approximately 11 km into a first of said range zones and
dividing all detached units having a transmission distance of
approximately 9 through approximately 20 km into a second of said
range zones.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to an optical TDM/TDMA (time
division multiplex/time division multiple suitable for use in a
communications system, and to a method for constructing and
operating such a TDM/TDMA system.
2. Description Of the Prior Art
Optical TDM/TDMA systems are known from "Optische
Ubertragungstechnik f ur fl achendeckende Teilnhmeranschl usse",
No. 04/1992, Section 5, pp. 9-12, No. 09/1992, Sections 9.0 and
9.1, pp. 24-29 and No. 10/1992, Sections 11.0 on pp. 4-6 and "Opal
94", pp. 11-13 of the periodical "Der Fernmelde-lngenieur" (Verlag
f ur Wissenschaft und Leben, Georg Heidecker GmbH, Erlangen).
What is referred to as the TDM/TDMA principle is often used in
optical transmission systems for wide-area subscriber lines. A
central station thereby sends message signals and control signals
in time-division multiplex (TDM) mode in a defined cycle via a
passive optical network that, as may be seen from FIG. 1 herein,
splits in the form of a light waveguide tree structure in the
direction from the central station OLT, to subscriber-proximate,
detached (remote) units ONU. Each remote unit is connected to a
combination of "plain old telephone system" (POTS) units and/or an
integrated services digital network (ISDN). In the opposite
direction, these detached units ONU transmit signal bursts
containing time-division multiplex signals with the same cycle back
to the central station OLT in time-division multiplex multiple
access (TDMA), i.e. periodically in a preselected sequence in
specific time slots. Separate light waveguide tree structures can
thereby be employed for the transmission of the signals from the
central station OLT and for the opposite direction; in view of
reducing outlay, however, operation in full duplex mode is often
accomplished with wavelength-division multiplex and only one light
waveguide tree structure referred to as a "passive optical network
(PON)". Compared to the reception time of the TDM signals, the
detached ONU units respectively return their signal bursts to the
central station OLT chronologically offset with a delay value that
is individually calculated for each detached ONU unit. The delay
value is selected for each detached unit ONU such that the signal
bursts of all detached units ONU arrive overlap-free and
frame-synchronized at the central station OLT after passing through
the various fiber lengths to the central station OLT. The control
and triggering of the delay thereby ensues by the central station
OLT on the basis of control signals co-communicated within the
time-division multiplex signals.
A frame format that is constructed basically the same for both
transmission directions and is composed of a plurality of
successive signal blocks is selected in a known way for the signal
transmission. A single signal block is shown in FIG. 2, line 1. The
signal block contains a control data part, also referred to as
frame overhead (FROH), provided with the address of the detached
unit ONU being addressed, this being followed by a comparatively
substantially longer useful data part ND. Control commands can be
sent from the central station OLT to the individual, detached units
ONU in the control data part (FROH). During a calibration (set-up
or "commissioning") procedure and for checking the running time
during operation, signals referred to as test packet signals that
contain a synchronization sequence for measuring transit time are
transmitted from the detached units ONU. The acknowledgement
signals for the control commands of the central station OLT as well
as status information of the detached unit ONU can be transmitted
from the detached unit ONU to the central station OLT.
The useful data for the respective detached unit ONU are
transmitted from the central station OLT in the useful data part
and the useful data of this detached unit ONU for the central
station OLT are transmitted in the opposite direction. The
transmission of control data thus reduces the transmission of
useful data, so that attempts are made to keep the control data
part FROH of the signal blocks as small as possible. This control
data part, however, also serves for the transmission of the test
packet signals in the periodically ensuing calibration or checking
of such a TDM/TDMA system, so that the control data part--as set
forth below--cannot be arbitrarily shortened.
For explanation, the calibration procedure of two detached units
ONU1, ONU5 of FIG. 1 shall be considered, these being arranged at
different distances from the central station OLT. It is assumed
that the detached unit ONU1 is immediately adjacent to the central
station OLT, so that a signal block transmitted from the central
station OLT according to line Z1 of FIG. 2 arrives in the detached
unit ONU1 after a negligible transit time as indicated by line Z2.
The reaction time to commands of the central station OLT is set
equal to a first basic delay GD1 by additional delay elements for
all detached units ONU, so that the detached unit ONU1 generates a
signal block according to line Z3 in response to a command of the
central station OLT after this basic delay, this signal block
according to line Z3 containing a test packet of measurement
signals that are arranged at the end of the control data part, for
measuring the transit time for the calibration procedure. This
signal block is returned to the central station OLT and arrives
thereat after a negligible transit time according to line Z4.
An anticipation window EWF is provided in the central station OLT,
having a length corresponding to the length of the control data
part FROH of the signal blocks and beginning with the end of the
transmission of the control data part by the central. The test
packets must arrive in this anticipation window during the
calibration procedure and during periodic checking. The beginning
of the anticipation window EWF for the test packets thus arises
from the minimum signal running time between the central station
OLT and the closest detached unit ONU, plus the first basic delay
GD1 that has been set. A transit time correction LZK must thus be
provided in the first detached unit ONU1 immediately adjacent to
the central OLT for supplementing the signal running time on the
optical fiber so that the signal block of this detached unit
arrives in the central station OLT in the specified position
corresponding to line 25.
Line Z6 shows a signal block that arrives after maximum signal
running time corresponding to the detached unit ONU5 located at the
maximum distance from the central station OLT. After the first
basic delay GD1, a signal block corresponding to line Z7 is
returned from the detached unit ONU5 to the central station OLT,
this signal block arriving exactly in the anticipation window EWF
of the central station OLT together with the control data part
corresponding to line Z8. A comparison to the specified position
for the signal block according to line Z9 shows that, by contrast
to line Z5, a correction of running time is not required and that,
moreover, the test packets of the control data part would no longer
be received in the anticipation window given an even longer running
time. An increase in the range of this system thus would require a
larger anticipation window, or a longer control data part of the
signal blocks, which would be at the expense of the useful data
transmission. As a result, however, the transmission capacity for
the useful data would be diminished in the overall system and the
data running time in the system and the outlay would be increased
because of the larger buffer memory for the useful data that would
then be required.
An increase in the range is easily possible if no detached units
ONU immediately adjacent to the central are present, i.e. the range
of distances is not increased. In this case, the differences in the
distance between central station OLT and detached units ONU would
only lead to differences in the signal running time that, however,
do not require an expansion of the anticipation window of the
central station OLT.
SUMMARY OF THE INVENTION
An object of the present invention is to improve the
above-described TDM/TDMA system such that an increase in the range
is achieved without reducing the transmission capacity for the
useful data and without lengthening the data running time.
In an optical TDM/TDMA system of the type initially cited, this
object is inventively achieved in that a system constructed and
operating in accordance with the principles of the present
invention wherein the detached units are divided into a plurality
of range zones, with all detached units in each range zone being
within a selected range of transmission distances from the central
station. All detached units in each range zone have delay means
therein for delaying emission of a TDMA signal block from that
detached unit, following reception of a TDM signal block, by a
delay which is dependent on the range zone which contains that
detached unit. All detached units in the range zone which is at the
largest transmission distance have the shortest delay. All detached
units in the other range zones respectively have delays which
become progressively longer as the transmission distance for the
range zone containing the detached unit becomes shorter. The system
is operated so as to introduce a frame offset between the
transmission cycle of TDM signal blocks emitted by the central
station and the transmission cycle of TDMA signal blocks emitted in
response thereto by the detached units. This frame offset is equal
to the duration of the anticipation window multiplied by the number
of range zones decremented by one. Moreover, initiation of the TDMA
transmission cycle, in reply to the TDM transmission cycle, does
not begin until completion of the reception cycle of the detached
units for the TDM transmission cycle.
As used herein, the term "transmission distance" means the actual
length or distance of optical fiber extending between the central
station and a particular detached unit. Since optical fibers cannot
always be laid in a "line of sight" line between the central
station and a detached unit, the transmission distance may differ
somewhat from the physical distance between the central station and
the detached unit.
The system of the invention has the advantage that the obtainable
range can be expanded to a multiple of the value, limited by the
length of the control data part of the signal blocks, and thus the
high output powers of transmission lasers that have now become
available can be exploited better. In a preferred fashioning of the
inventive system wherein the detached units are divided into two
range zones, and wherein the delay of the second range zone is
selected so that the signal block emitted by each detached unit in
the second range zone arrives at the central station just at the
beginning of the anticipation window, taking into account the
aforementioned frame offset, a doubling of the range is
achieved.
DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the basic components a TDM/TDMA system having a
passive optical network and detached units arranged at different
distances from the central station constructed and operated in
accordance with the principles of the present invention;
FIG. 2 as discussed above, illustrates a known calibration or
checking procedure for a conventionally constructed TDM/TDMA
system;
FIG. 3 illustrates a calibration or checking procedure for the
system of FIG. 1 in accordance with the principles of the present
invention in a first range of distances;
FIG. 4 illustrates a calibration or checking procedure in a second
range of distances for the system of FIG. 1 in accordance with the
principles of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1, the central state OLT of the TDM/TDMA system is
connected to the local exchange V, this central station OLT, in
addition to containing control and evaluation devices, contains a
laser transmitter that feeds the optical signals intended for
transmission to the detached units ONU1 . . . ONU5 into the passive
optical network PON. The first detached unit ONU1 is located
immediately adjacent to the central station OLT and the second
detached unit ONU2, whose internal circuitry is shown as an example
of all of the detached units ONU1 . . . ONU5, is at a somewhat
longer transmission distance from the central station OLT. The
third through fifth detached units ONU3, ONU4 and ONU5, are located
at an even longer transmission distance from the central station
OLT. Each detached unit is connected to standard telephones POTS or
to broadband telephone equipment ISDN allocated to individual
subscribers. (It will be recognized that although five detached
units are shown in this exemplary embodiment, the invention is
applicable to any system having two or more such detached units,
with at least one unit in each range zone.)
The internal circuitry of the second detached unit ONU2 shown as an
example of all detached units contains a beam splitter SPL
connected to the optical fiber input. This beam splitter SPL
supplies the incoming light pulses to an opto-electrical transducer
and feeds the outgoing optical pulses into the passive optical
network PON. The useful signal receiver NSE and the control signal
receiver STE are connected to the opto-electrical transducer. The
output of the useful signal receiver NSE is connected to a
subscriber line TAS that, in the present example, emits the
received useful signals to an ISDN telephone and forwards signals
generated by the latter to the laser transmitter SEN. There is a
direct control connection between control signal receiver STE and
laser transmitter SEN; a delay element T through which the useful
data also pass is additionally inserted into the laser transmitter
SEN. The delay element T is connected to the control signal
receiver STE by an additional control line and thus the delay
thereof can be switched (adjusted) proceeding from the central
station OLT.
The calibration procedure for a conventional TDM/TDMA system has
already been set forth in conjunction with FIG. 2. Even though
certain of the components of FIG. 1 were used for exemplary
purposes as "generic" units in that discussing none of the
inventive features of FIG. 1 were employed.
In the TDM/TDMA system of the invention, the detached units ONU1 .
. . ONU5 are divided into two range (transmission distance) zones,
whereby the detached units of the range zone 1 are located at a
transmission distance between 0 and 11 km and the detached units of
the range zone 2 are located at a transmission distance of
approximately 9 through approximately 20 km from the central
station OLT.
The calibration or checking procedure for the detached unit ONU1
located in the range zone 1 shall be set forth in conjunction with
FIG. 3. Line Z1 of FIG. 3 shows a signal block sent from the
central station OLT to one of the detached units ONU, this signal
block corresponding to the signal blocks of FIG. 2. Line Z2 of FIG.
3 shows this signal block upon reception in the first detached unit
ONU1 immediately adjacent to the central; because of the negligible
fiber running time, the temporal position of the two signal blocks
is substantially identical. Line Z3 shows a signal block in the
temporal position in which it is sent from the first detached unit
ONU1 to the central. The frame, and thus the start of these signal
blocks, has been shifted forward by a time corresponding to a frame
offset RO between the transmission cycle and the reception cycle of
the detached units. This frame offset RO is valid for all detached
units of the inventive TDM/TDMA system and is calculated from the
length of the anticipation window EWF of the central multiplied by
the number of range zones decremented by one.
In the range zone 1, the delay element T of each detached unit is
set to a second basic delay GDB1 that represents the sum of the
first basic delay GD1 and the frame offset RO according to FIG. 3.
The position of the transmission time of the first detached unit
ONU1 according to line Z3 thus corresponds to the reception time in
the central station OLT according to line Z4 and the specified
position according to line Z5 again corresponds to those of FIG.
2.
The third detached unit ONU3 is located at a distance of
approximately 11 km fiber length from the central station OLT, so
that the signal blocks shown in line Z6 arrive with a time shift.
The time shift corresponding to the frame offset RO and the second
basic delay GDB 1 is again valid for the transmission of the signal
blocks corresponding to line Z7 by the third detached unit ONU3.
After a running time of approximately 110 microseconds for a link
having a fiber length of 11 km, the signal block is received in the
central station OLT in the position shown in line Z8, which
directly corresponds to the specified position of line Z9. It thus
follows that no additional correction of running time is required
for the third detached unit ONU3, whereas the first detached unit
ONU1 requires and additional running time correction LZK so that
the test packet signals in the useful signal transmission arrive in
the anticipation window EWF of the central station OLT. Controlled
by the latter, the delay element T of the first detached unit ONU1
is switched according to the correction in running time that has
been calculated.
Line Z1 in FIG. 4 again shows a signal block that is transmitted
from the central station OLT to the fourth detached unit ONU4
located at 9 km fiber length from the central station OLT and that,
according to line Z2, arrives after a corresponding running time.
It is also valid for the detached units in the range zone 2 that
the transmission cycle to the central station OLT, and thus the
frame for the signal blocks, begins one frame offset RO earlier
compared to the frame of the signal blocks received from the
central station OLT. The basic delay GDB2 for the range zone 2 is
selected such that the test packet still just falls at the
beginning of the anticipation window EWF with the given frame
offset at the lower limit of the range zone. A lead in the output
of the signal blocks from the detached units ONU4 and ONU5 arranged
in the range zone 2 thereby arises. The signal blocks sent from the
respective detached unit to the central station OLT during the
reception of the signal blocks of the signals cannot yet contain
the response to control instructions of the signal blocks; this
response ensues at the earliest in the transmission cycle,
(specifically, the signal block thereof) following the respective
reception cycle for all detached units of the inventive TDM/TDMA
system.
Line Z4 of FIG. 3 shows the signal block sent by the fourth
detached unit ONU4 upon reception in the central station OLT, i.e.
after a running time corresponding to a fiber length of 9 km. It
can be seen that the test packet signals of this signal block still
just occur in the anticipation window EWF of the central station
OLT, i.e. line Z4 and line Z5 as well of FIG. 4 correspond to lines
Z4 and Z5 of FIG. 3, so that a running time correction LZK is also
to be inserted in the fourth detached unit ONU4 as in the first
detached unit ONU1. This results in the overlap of the range zones
being uncritical, so that no complicated measurements are required
before the calibration or checking procedure for the division into
range zones.
Line Z6 of FIG. 4 shows the position of a signal block received in
the fifth detached unit ONU5. This detached unit is located at the
maximum distance of approximately 20 km fiber length from the
central station OLT, so that the signal block is received
correspondingly delayed. During assembly, a time shift
corresponding to a frame offset RO having the length (duration) of
an anticipation window multiplied by the number of range zones
decremented by one is also preset in the fifth detached unit ONU5.
Corresponding to the allocation to the range zone 2, the basic
delay GDB2 for the second range zone is set, just as for the fourth
detached unit ONU4, so that the fifth detached unit ONU5 already
begins sending its signal blocks to the central station OLT before
the beginning of the reception of the signal blocks sent from the
central station OLT. After a running time corresponding to a fiber
length of 20 km, these signal blocks are received by the central
station OLT corresponding to line Z8; comparison to line Z9 shows
that a reception at the specified position is thereby achieved.
The TDM/TDMA system of the invention only contains detached units
ONU in two range zones, however, it can easily be seen that a
division into three range zones having correspondingly graduated
frame offset RO is easily possible for a further increase of the
range.
Although modifications and changes may be suggested by those
skilled in the art, it is the intention of the inventor to embody
within the patent warranted hereon all changes and modifications as
reasonably and properly come within the scope of his contribution
to the art.
* * * * *